I thought the 275s would have made it on a 10" rim with that 4.5" backset. She is going to be sitting pretty with the 245s and if there is some room, I'll bump up to 255
A 275/60/15 isn't even recommended for a 10" wide rim, 9.5" is the widest recommended for that. Not to mention finding a 15x10" rim with a 4.5" backspace, that's a -26mm offset. A 15x9 with 4.5" of backspace would probably work, with 5-9/16" to the springs a 1/2" of sidewall overhang and 9/16" of spring clearance puts you at 4.5" of backspace. That would probably be tight on the quarters. Remember that a rims are an inch wider outside-to-outside than advertised, the advertised width is inside to inside where the tire mounts. So a 15x10" rim is actually 11" wide outside lip-to-outside lip, a 15x9" is 10" wide, 15x8" is 9" wide, etc.
The 15x8's would work just fine, that's actually the width used by the manufacturer to measure the tire specs for a 275/60/15. You would just need a backspace between 4" and 4.125".